This invention relates to electronic ballasts for gas discharge lamps and, in particular, to an electronic ballast that shuts off filament current after starting.
A fluorescent lamp is an evacuated glass tube with a small amount of mercury in the tube. The tube is lined with an adherent layer of a mixture of phosphors. Some of the mercury vaporizes at the low pressure within the tube and a filament or cathode in each end of the tube is heated to emit electrons into the tube, ionizing the gas. A high voltage between the filaments causes the mercury ions to conduct current, producing a glow discharge that emits ultraviolet light. The ultraviolet light is absorbed by the phosphors and re-emitted as visible light.
A fluorescent lamp is a non-linear load to a power line, i.e. the discharge current through the lamp is not directly proportional to the voltage across the lamp. Current through the lamp is zero until a minimum voltage is reached, then the lamp begins to conduct. Once the lamp conducts, the discharge current will increase rapidly unless there is a ballast in series with the lamp to limit current.
An electronic ballast is a small power supply having the fluorescent lamp as a load. The ballast typically includes a rectifier for converting alternating current (AC) from a power line to direct current (DC) and an inverter for changing the direct current to alternating current at high frequency, typically 25-60 kHz. Some ballasts include a boost circuit between the rectifier and the inverter for increasing the DC voltage, e.g. from approximately 180 volts (assuming a 120 volt input) to 300 volts or more.
There is increasing pressure on the lighting industry and by the lighting industry to improve the efficiency of lighting systems. The converter-inverter stages of a ballast have been refined over the last few years to substantially improve the efficiency of electronic ballasts. Lamp designs have also improved, including the recent introduction of a new family of "T5" lamps. The designation "T5" refers to the diameter of the glass tube of the lamp, as measured in eighths of an inch. In general, a narrower lamp is more efficient than a wider lamp at producing light.
In measuring the efficiency of a lamp, one could consider only the electrical characteristics of the glow discharge relative to the amount of light produced. However, it is the efficiency of the system that must be improved, which means that any power dissipation that does not directly contribute to the production of light must be eliminated or at least reduced. For many fluorescent lamps, including the new T5 lamps, this includes the power dissipated in the filaments. Lamp manufacturers are increasingly specifying a maximum terminal current into a lamp. Such a specification is difficult to meet without removing filament drive when a lamp is operating normally.
When a lamp is started, current is provided to the filaments or heaters in each end of the lamp. The filaments become red hot, which substantially increases the emission of electrons and greatly facilitates starting the lamp. When a lamp is operating normally, the filament current can be reduced and many circuits have been proposed for reducing filament current after a successful start; for example, see U.S. Pat. Nos. 4,935,669; 5,015,923; 5,027,032; 5,179,326; and 5,256,939.
Another problem is that many lamps are dimensionally the same but are electrically quite different. Inevitably, someone will connect the wrong lamp to a ballast. One can provide a ballast that monitors lamp voltage as an indication of lamp type but there are ballasts on the market that can be connected to either one lamp or two lamps (of a particular type). There is a growing expectation in the market that all ballasts can be used with either one lamp or two lamps. Thus, lamp voltage alone is not enough. Further, fault detectors within a ballast must be able to distinguish the absence of a second lamp from a fault condition.
In view of the foregoing, it is therefore an object of the invention to provide an electronic ballast that can distinguish lamp types.
Another object of the invention is to provide a ballast that can be used, without adjustment, for either a single lamp or with two lamps.
A further object of the invention is to distinguish different types of T5 lamps automatically.
Another object of the invention is to provide an electronic ballast that has higher efficiency than electronic ballasts of the prior art.
A further object of the invention is to provide an electronic ballast having a filament cutout circuit that consumes very little power compared to the filaments.